Fig. 143.—Veneridæ
1. Venus fasciata. 2. Venus striatula. 3. Tapes virgineana. 4. Tapes aurea
The same family (Veneridæ) contains the large genus Tapes, so called because many of its shells are marked in such a manner as to recall the patterns of tapestry. The general form of these shells is oblong, and the margins are quite smooth. They are frequently washed up on the beach, especially during storms, but the animals may be found alive at low water, buried in sand, or hiding in the crevices of rocks or among the roots of the larger sea weeds. The mantle is open at the anterior end, and the siphons are either quite distinct or only partly united.
Some of the shells are very prettily coloured. One (T. aurea) receives its name from the yellow ground, which is variously marked by deeper tints; another (T. decussata) is so called on account of the cross grooves with which the shell is sculptured; and a third (T. virgineana), which inhabits the muddy bottoms of deep water, is prettily marked by radiating bands that run from the umbones to the ventral margins.
We now come to the family Cyprinidæ, in which the shell is regular in form, oval or elongated; and the valves, which are equal in size, are thick and solid, and fit closely. The teeth are beautifully formed, the central ones numbering from one to three in each valve, and the pallial line is not sinuated. The mantle lobes are united on the posterior side by means of a kind of curtain that is pierced by two siphonal openings. There are two gills on each side, united posteriorly, and the foot is tongue-shaped and thick.
The typical genus—Cyprina—contains a large mollusc (C. islandica), which is moderately common round our shores, especially in the north, but is not often seen above low-water mark, except when washed up by storms. The shell is oval and thick, with the umbones prominent and turned towards the posterior side, and the ligament is strong and prominent. It is entirely covered with a thick epidermis, of a rich brown colour, often exhibiting a fine silky gloss, especially near the margins. The interior of the shell is white, and the adductor impressions oval and polished.
The same family includes some smaller shells that inhabit deep water, and are therefore not commonly seen on the beach. Among these are two species of the genus Astarte, one of which is deeply furrowed in a direction parallel with the margins; also Circe minima, which seldom exceeds half an inch in length. Although so small compared with Cyprina, these shells may be identified by their clothing of epidermis, together with the family characteristics given above.
The Cyprinidæ also contains the interesting Heart Cockle (Isocardia cor), the form of which is so characteristic that identification is easy. The heart-shaped shell is thick and strong, and is swollen out in such a manner that the umbones are wide apart. These latter are also curved into a spiral form, and the ligament between them is prominent. The colour of the shell is variable, the epidermis being of any shade from a yellow to a dark brown. The foot is small and pointed, and the siphons fringed.